首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   265篇
  免费   30篇
  国内免费   43篇
测绘学   32篇
大气科学   59篇
地球物理   23篇
地质学   121篇
海洋学   40篇
天文学   1篇
综合类   18篇
自然地理   44篇
  2024年   1篇
  2023年   7篇
  2022年   6篇
  2021年   5篇
  2020年   7篇
  2019年   4篇
  2018年   2篇
  2017年   16篇
  2016年   7篇
  2015年   12篇
  2014年   18篇
  2013年   13篇
  2012年   9篇
  2011年   16篇
  2010年   10篇
  2009年   14篇
  2008年   18篇
  2007年   18篇
  2006年   22篇
  2005年   5篇
  2004年   26篇
  2003年   15篇
  2002年   14篇
  2001年   14篇
  2000年   16篇
  1999年   13篇
  1998年   5篇
  1997年   7篇
  1996年   4篇
  1995年   6篇
  1993年   4篇
  1992年   1篇
  1990年   2篇
  1989年   1篇
排序方式: 共有338条查询结果,搜索用时 203 毫秒
181.
总结了我国科技工作者在食用菌种质资源、育种、栽培和加工技术等方面取得的研究进展,提出我国食用菌的发展方向:加强生物技术育种研究和应用、重视野生食用菌的人工驯化栽培研究、加强生物多样性保护力度、快速推进精深加工技术及其产业化研究、加强质量标准和检测体系建设。  相似文献   
182.
This paper presents an account of the progress of the integration between the Danish and Swedish parts of the Öresund Region within the world of science, with emphasis on biotech. The paper is based on data from the database Science Citation Index Expanded. The amount of co-authorships produced by agents from each side of the region is analysed for a 12-year period (1994–2005) and used as an indicator to show how the integration is proceeding. Each of the co-authorships is examined to shed light on the nature of the interactions, thereby identifying the agents most involved in collaboration. Furthermore, the development in co-authorships within the region is compared to the development in co-authorships between the Öresund Region and four selected reference areas. Our findings indicate that there has been a substantial increase in both the number of co-authorships overall and within biotech solely. The rise in co-authorships between the two parts is proceeding at a faster pace than the rise in co-authorships between the Öresund Region and the reference areas. The overall results of this paper indicate a growing integration of the biotech sector in the Öresund Region.  相似文献   
183.
Neil Wrigley 《Geoforum》2006,37(3):340-351
This paper examines the organizational challenge of e-commerce for emerging retail transnational corporations (TNCs). The paper begins by scoping and conceptualizing the retail TNCs and in the process, draws attention to the high level of territorial embeddedness of these firms. This provides the framework for the remainder of the paper, where the organizational challenge of a ‘multi-channel’ paradigm (blending online and offline forms of retail distribution) is evaluated. Specifically, the paper argues that place, space and embeddedness continue to matter in the ‘new e-conomy’ of multi-channel distribution.  相似文献   
184.
中国煤层气勘探开发的进展与趋势   总被引:6,自引:0,他引:6  
秦勇  程爱国 《中国煤田地质》2007,19(1):26-29,32
中国目前处于从煤层气资源大国转型为生产大国的关键时期,煤层气勘探与开发活动取得突破,但产业发展仍存在诸多问题。通过总结我国近期煤层气勘探开发活动的进展情况,指出了产业发展面临的主要技术问题,阐述了国家和国内主要煤层气企业今后五年的发展趋势,以期对我国煤层气勘探开发活动有所促进。  相似文献   
185.
王明荣  朱春林 《云南地质》2007,26(2):213-218
经对多个滑坡体勘察实践,结合前人研究成果,对滑动带(面)的确定,滑带土抗剪强度指标的选取,稳定性计算中易被忽视的问题进行探讨。  相似文献   
186.
The well-known “Maximum Entropy Formalism” offers a powerful framework for deriving probability density functions given a relevant knowledge base and an adequate prior. The majority of results based on this approach have been derived assuming a flat uninformative prior, but this assumption is to a large extent arbitrary (any one-to-one transformation of the random variable will change the flat uninformative prior into some non-constant function). In a companion paper we introduced the notion of a natural reference point for dimensional physical variables, and used this notion to derive a class of physical priors that are form-invariant to changes in the system of dimensional units. The present paper studies effects of these priors on the probability density functions derived using the maximum entropy formalism. Analysis of real data shows that when the maximum entropy formalism uses the physical prior it yields significantly better results than when it is based on the commonly used flat uninformative prior. This improvement reflects the significance of the incorporating additional information (contained in physical priors), which is ignored when flat priors are used in the standard form of the maximum entropy formalism. A potentially serious limitation of the maximum entropy formalism is the assumption that sample moments are available. This is not the case in many macroscopic real-world problems, where the knowledge base available is a finite sample rather than population moments. As a result, the maximum entropy formalism generates a family of “nested models” parameterized by the unknown values of the population parameters. In this work we combine this formalism with a model selection scheme based on Akaike’s information criterion to derive the maximum entropy model that is most consistent with the available sample. This combination establishes a general inference framework of wide applicability in scientific/engineering problems.  相似文献   
187.
毕岭  肖玲  贾慧  章宏波  陈曦 《地质论评》2023,69(6):2335-2347
遗迹化石是研究古环境、古气候、地层旋回性及其全球成因动力学研究的有效载体。遗迹化石是某种环境条件下生物行为习性的直接证据,是沉积环境的灵敏指示剂,对探讨生物行为习性的演化、生物的多样性、生物地层学、事件地层学、储层地质学等都具有重要的理论意义和应用价值。基于此,笔者等选取CNKI核心合集数据库中的数据,对1954~2022年遗迹化石相关文献进行系统整理,采用Microsoft Excel 2016和CiteSpace 6.1.R2 软件对文献类型分布、年度发文数量、国家、机构、期刊、作者、高引文献、突现关键词等进行分析,构建和绘制各单元间相互关系,以探求研究领域关键路径及知识拐点,进而挖掘所蕴含的新知识,预测学科研究趋势。研究表明:①我国遗迹化石领域研究发文量最多的机构是河南理工大学;发文量最多的学者是胡斌;被引频次最多的论文作者是龚一鸣;我国遗迹化石的研究主要以国家层面的基金资助为主。②系统发展阶段,我国遗迹化石研究区主要集中在贵阳、新疆;创新发展阶段,研究区主要集中在河南、四川等地。③中国寒武系的遗迹化石研究主要集中在河南省、云南省和贵州省境内;奥陶系、志留系和泥盆系中的遗迹化石研究主要集中在贵州、陕西、新疆塔里木盆地、准噶尔盆地、湖南等地。石炭系—二叠系的遗迹化石研究主要集中在新疆准噶尔盆地西北部、柴达木盆地南缘等地;三叠系的遗迹化石研究主要集中在扬子地区。笔者等基于新的研究思路和方法,系统分析了国内遗迹化石研究现状和趋势,对于进一步了解遗迹化石具有指导意义。  相似文献   
188.
通过梳理2018年8月在美国纽约举行的第十届城市气候国际会议主要研究成果,分类汇总了城市气象研究的最新国际进展与发展趋势,包括城市气候过程、城市环境下气候变化适应与应对、气候条件下城市规划和管理、最新城市气象探测技术、城市环境数值模拟、大城市气候、城市遥感、城市地区极端天气等方面。在此基础上,根据我国城市化发展特性,探讨了我国城市气象研究的未来发展方向。  相似文献   
189.
Conservation actions for sharks are hampered partly by the negative beliefs and inaccurate knowledge about them among the general public. This paper investigated the knowledge and attitude of residents and tourists towards shark species of Fernando de Noronha Archipelago, a marine protected area, and one of the major ecotourism destinations in Brazil. Findings showed that residents had a lower knowledge of sharks and less positive attitudes towards sharks than tourists. Even though residents did not see sharks as threats to humans and they were aware of the role of the archipelago as a shark nursery, they did not completely understand the sharks' ecological and economic importance. This distinction between residents' and tourists' perceptions of sharks demands different strategies to assure engagement in management and conservation policies relating to both groups. Management actions focusing on providing enjoyable experiences with sharks to residents and enhancing their participation in tourist activities related to sharks are presented. They would require innovative cooperation between scientific, management and environmental institutions to build initiatives aimed at increasing the knowledge and improving attitudes of residents towards shark conservation and to homogenise conservation values held by the entire public on the archipelago.  相似文献   
190.
Fe是海洋“生物泵”中限制浮游生物生长和控制海洋初级生产力的主要因素之一,也可间接影响大气中CO2含量,反馈于全球的气候变化。近年来基于多接收电感耦合等离子体质谱仪(MC ICP MS)分析方法的改进及测试精度的提高,应用Fe同位素组成、变化及其分馏机制,为研究海水中Fe的主要来源以及示踪海洋环境中Fe的循环过程等,提供了一个有效地球化学指标,也对示踪地球不同演化阶段的海洋沉积环境变化具有指示意义。较为详细地介绍了海洋环境中不同储库的Fe同位素组成,洋中脊热液流体—玄武岩、海水—大洋玄武岩等水—岩反应影响Fe同位素分馏效应的主要因素及地球不同演化阶段古海洋沉积环境中的Fe同位素变化。认为海洋环境下Fe同位素可以产生较为明显的分馏作用,轻铁同位素具有更易活动、易迁移的特征,并进一步提出不同相态、不同矿物间Fe同位素分馏系数的确定等相关问题仍是今后Fe同位素研究的主要方向。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号